How jellyfish create stinging water

Novel structures formed of stinging cells found in the mucus of the upside-down jellyfish Cassiopea xamachana are described in a paper published in Communications Biology. These structures - named cassiosomes - are capable of killing prey and likely cause the sensation known as ‘stinging water’, experienced by snorkelers in mangrove forest waters.

Mangrove forests are home to Cassiopea jellyfish, which lie upside-down on the bottom of shallow waters with frilly oral-ams facing upward. Snorkelers in these waters around Florida, the Caribbean and Micronesia have long reported the feeling of jellyfish stings without coming into direct contact with any jellyfish. Many possible explanations for this stinging water have been suggested, but the exact cause is unknown.

Gary Vora and colleagues reviewed the scientific literature on Cassiopea dating back to the early 1900s to search for clues about stinging water. They learned that mucus released by Cassiopea contains tiny cell masses, called cassiosomes. The authors used microscopy and found that the outer layer of the cassiosome is lined with thousands of jellyfish stinging cells called nematocysts. Nematocysts are toxin-filled capsules normally found in specialized cells in jellyfish tentacles. The authors show that jellyfish release the cassiosome-filled mucus into the water like tiny grenades, thus creating stinging water. The authors also found these cassiosomes in four additional related species of jellyfish.